Allow to set to expand aiomysql.cursors.Cursor

[elBroom]

In #263 the custom cursor was removed.
For each query I need to set a comment with 'request_id' and measure time of the query. Initially, I thought to do this through before_cursor_execute, but the library does not support subscribing to events. Now, I think to expand the cursor aiomysql.cursors.Cursor.

[aio-libs-bot]

GitMate.io thinks possibly related issues are #251 (cursor.nextset() hangs if the next set has an error), #214 (get a problem in cursor), #158 (aiomysql enabled cache?), #159 (how to use aiomysql with tornado?), and #303 (aiomysql Import Error _scramble).